1. Field of the Invention
The present invention relates to the field of semiconductor circuits and, in particular, to a method and system for improving the manufacturing process of an integrated circuit (IC).
2. Description of the Related Art
In an IC fabrication process, optical lithography using one or more photomasks is applied to transfer designed circuit patterns onto a given wafer layer (e.g., active area, transistor gate polysilicon, contact, Metal-1, etc.) of an integrated circuit (IC). These circuit patterns represent layout features of the IC. Over the past few years, the dimensions of the layout features in ICs have been reduced to achieve greater circuit operating frequencies and increased integration densities. With reduced dimensions, advanced photomasks have been used to enhance fidelity of the manufactured circuit patterns to the designed circuit patterns on each wafer layer of the IC. Advanced photomasks embody Reticle Enhancement Techniques (RETs), such as sub-resolution assist features (also known as “scattering bars”), optical proximity correction, alternating-aperture phase-shifting, and the like. These techniques compensate for changes in the size and shape of the pattern that occur during the pattern transfer from the photomask(s) to the wafer layer. These changes are due to various manufacturing non-idealities, including optical effects such as scattering of light, as well as focus variations (i.e., variations in the distance between a lens and the wafer layer).
Present-day RET techniques assume a constant value of focus variation (for example, “zero defocus” corresponds to ideal or nominal focus conditions) for all RET calculations for features on the wafer layer. However, with advanced lithographic processes and small feature dimensions, imperfect planarity—i.e., topographic variation—of wafer layers has become a major cause of focus variation in the succeeding layers to be manufactured. The wafer topographic variation occurs both systematically and randomly within the layout region of the IC; in particular, the systematic component of such variation is reproduced in each copy of the IC on the wafer. The wafer topographic variation also occurs in spite of such manufacturing techniques as chemical-mechanical planarization (CMP), since the result of such techniques varies with the local layout pattern of the IC. Here, locality is at length scales of between several tenths of a micron and several thousand microns. In other words, the post-CMP topographic height of the wafer depends on the layout pattern context out to a radius of from tenths to thousands of microns. The thickness or underlying topographic variations of a given wafer layer of the IC are not only highly significant relative to the focus control that is required for accurate lithography, but are also largely systematic and predictable.
One method to reduce the topographic variations within a given IC is the technique of dummy fill insertion. Dummy fill insertion entails the addition of non-functional, extra (“dummy”) shapes to the originally designed layout, so as to enhance the uniformity of layout feature density and hence the uniformity of planarization of a wafer layer. However, even after dummy fill insertion, many topographic variations remain in the wafer layer within the boundary of the IC. These topographic variations, by causing variations in focus, affect the critical dimensions of the layout features that are to be patterned on the wafer layer. The critical dimensions of a given layout feature include the length and width of the feature.
U.S. Pat. No. 6,673,638, titled “Method and Apparatus for the Production of Process Sensitive Lithographic Features”, filed on Jan. 28, 2002, and assigned to KLA-Tencor Corporation, proposes defocus test structures on a reticle to model variations of critical dimensions of a circuit pattern to be printed on a wafer layer. This modeling is done with respect to the shape and spatial parameters of an IC layout.
U.S. Pat. No. 6,194,104, titled “Optical Proximity Correction Method for Improving Lithography Process Window”, filed on Oct. 12, 1999, and assigned to Taiwan Semiconductor Manufacturing Company, proposes a method for optical proximity correction (OPC) of photomask aperture shapes used for photolithographic patterning. The OPC shapes are derived by mathematically modeling contours to be printed on a wafer layer.
A publication titled “Using OPC to Optimize for Image Slope and Improve Process Window”, by N. B. Cobb and Y. Granik, and published in SPIE Int. Soc. Opt. Eng. 5130, 838, 2003, proposes a method to solve for OPC at a non-zero defocus value to increase depth of focus. The method minimizes an objective that is a function of edge placement error, i.e., the error that occurs while transferring the pattern to a wafer layer.
The methods and systems cited above do not take into account systematic and predictable post-planarization topographic variations of the wafer surface. Yet, such wafer topographic variations affect focus in a lithography process which in turn affects patterning accuracy and manufacturing yield—both functional and parametric—of the IC. In light of the above discussion, there is a need for a system and method that will account for topographic variations in a wafer layer when calculating RETs such as sub-resolution assist features (SRAFs) or OPC. Further, the layout of any given IC layer should be optimized based on the predicted effects of wafer topography on focus and lithographic patterning accuracy.